In an air-cooling heat pump chiller, a heat exchanger on a heat source side becomes frosted in a heating operation under a condition that an external temperature is low. Further, if the frosting progresses, it hinders heat exchange which leads to deterioration of its efficiency. Accordingly, a defrost operation for removing frost is necessary. In this case, in the heat pump chiller, a water heat exchanger on a user side functions as an evaporator to take heat, and there is a problem in that a temperature when heat is supplied to the load is reduced.
Thus, in a system in which plural heat pumps are connected with respect to a load, when a specific heat pump becomes frosted and starts a defrost operation, a technique of forcibly operating a stopped heat pump or operating a preliminary heat pump which is under an abnormal operation to back up a decreased capacity is disclosed in PTL 1 and PTL 2. Further, PTL 3 discloses a technique of preventing two or more heat pumps among plural heat pumps which are connected with respect to a load from starting a defrost operation in the same period of time.
Meanwhile, in recent years, a heat pump has been modularized in a unit. For example, a heat pump system in which an individual air-cooling heat pump chiller is modularized, plural modules are connected in series to form a unit, and plural units are connected in parallel with respect to an external load (system load) has been practically used.